User guide

Neuron C Programmer’s Guide 9
declared with the polled modifier), the new value of the network variable is
propagated across the network to all devices with input network variables
connected to that output network variable. If the output network variable is not
currently a member of any network variable connection, no transaction and no
error occurs.
Although the propagation of network variables occurs through L
ONWORKS
messages, these messages are sent implicitly. The application program does not
require any explicit instructions for sending, receiving, managing, retrying,
authenticating, or acknowledging network variable updates. A Neuron C
application provides the most recent value by writing into an output network
variable, and it obtains the most recent data from the network by reading an
input network variable.
Example:
network input SNVT_temp nviTemperature;
network output SNVT_temp nvoTemperature;
void f(void)
{
nvoTemperature = 2 * nviTemperature;
}
Network variables greatly simplify the process of developing and installing
distributed systems because devices can be defined individually, then connected
and reconnected easily into many new L
ONWORKS applications. Network
variables are discussed in detail in Chapter
3, How Devices Communicate Using
Network Variables
, on page 43, and also in the Neuron C Reference Guide .
Network variables promote interoperability between devices by providing a well-
defined interface that devices use to communicate. Interoperability simplifies
installation of devices into different types of networks by keeping the network
configuration independent of the device’s application. A device can be installed in
a network and logically connected to other devices in the network as long as the
data types (for example, SNVT_switch or SNVT_temp_p) match. To further
promote interoperability, the L
ONWORKS platform provides standard functional
profiles that define standard functional interfaces for devices, and standard
network variable types (SNVTs) that define standard data encoding, scaling, and
units, such as degrees C, volts, or meters. There are standard functional profiles
for a variety of functions and industries. There are SNVT definitions for
essentially every physical quantity, and other more abstract definitions tailored
for certain industries and common applications.
You can also create your own user functional profiles and user network variable
types (UNVTs). You can define resource files for your custom types and profiles
to enable your devices to be used with devices from other manufacturers. The
NodeBuilder Resource Editor included with the NodeBuilder tool provides a
simple interface for viewing existing resources and defining your own resources.
Configuration Properties
A configuration property is a data item that, like a network variable, is part of
the device interface for a device. A configuration property can be modified by a
network tool. Configuration properties facilitate interoperable installation and
configuration tools by providing a standardized network interface for device